Intermittent site offline

Hi all. I have a number of sites, all of which suffer from going offline (redirect loop) on the front end only intermittently and at different times. I am using Cloudflare flexible SSL, with the Cloudflare redirect SSL plugin and/or the standard Cloudflare plugin. I am always able to log in, and purging the cache ( nginx helper) solves the front end problem. The logs don’t show much. What on earth could this be? TIA